1. Technical Field of the Invention
The present invention relates to a terahertz wave generating method and apparatus that have a first laser beam and a second laser beam incident on a difference frequency mixing unit and overlap the two laser beams in the difference frequency mixing unit, thereby generating a terahertz wave whose frequency is a frequency difference between the two laser beams.
2. Description of the Related Art
Terahertz waves are electromagnetic waves that lie between radio waves and infrared radiation, and are 0.3 THz to 10 THz in frequency and 30 μm to 1 mm in wavelength. Such terahertz waves have penetrability to various substances including paper and plastics like radio waves, and also have a proper spatial resolution like light. In addition, terahertz waves have a unique absorption spectrum according to substance. Therefore, for example, such an application that identifies a type of a substance concealed in a parcel and the like and detects a dangerous article such as an explosive by spectrometry can be expected. To realize this, a terahertz wave generating apparatus that generates a terahertz wave at various frequencies is essential.
Conventionally, a technique of exciting a difference frequency mixing unit with laser light is widely employed for a terahertz wave generating apparatus suitable for spectrometry. This technique can mainly be classified into the following two types.
(Single Frequency Generation by Difference Frequency Mixing)
A terahertz wave of a single frequency is generated by a principle of difference frequency mixing, using two single-frequency laser beams that differ in frequency and wavelength (e.g. Patent Document 1 listed below). Moreover, the frequency of one of the laser beams is controlled to change the frequency difference, thereby changing the frequency of the terahertz wave.
(Broadband Generation by Femtosecond Laser)
There is a technique of generating a broadband terahertz wave pulse using a broadband femtosecond pulse laser (e.g. Patent Document 2 listed below).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-341392
[Patent Document 2] Japanese Patent Application Laid-Open No. 2002-223017
In the above-mentioned “single frequency generation by difference frequency mixing”, there is a problem that, when a laser beam wavelength is restricted, laser intensity and terahertz wave output decrease, too. There is also a problem of low stability because the frequency of the laser itself is controlled.
In the above-mentioned “broadband generation by femtosecond laser”, a terahertz wave of various frequency components can be obtained at one time. However, since all frequency components are included in one pulse, the frequency components need to be divided by a Fourier transform in order to obtain spectroscopic information.